This invention relates to implantable medical devices (IMDs), particularly methods and apparatus for electrically isolating and supporting component sub-assemblies formed of multiple components in volumetrically efficient ways.
A wide variety of IMDs are known in the art. Of particular interest are implantable cardioverter-defibrillators (ICDs) that deliver relatively high-energy cardioversion and/or defibrillation shocks to a patient""s heart when a malignant tachyarrhythmia, e.g., atrial or ventricular fibrillation, is detected. The shocks are developed by discharge of one or more high voltage electrolytic capacitor that is charged up from an ICD battery. Current ICDs typically possess single or dual chamber pacing capabilities for treating specified chronic or episodic atrial and/or ventricular bradycardia and tachycardia and were referred to previously as pacemaker/cardioverter/defibrillators (PCDs). Earlier developed automatic implantable defibrillators (AIDs) did not have cardioversion or pacing capabilities. For purposes of the present invention ICDs are understood to encompass all such IMDs having at least high voltage cardioversion and/or defibrillation capabilities.
Energy, volume, thickness and mass are critical features in the design of ICD implantable pulse generators (IPGs) that are coupled to the ICD leads to form the completed ICD. The battery(s) and high voltage capacitor(s) used to provide and accumulate the energy required for the cardioversion/defibrillation shocks have historically been relatively bulky and expensive. Presently, ICD IPGs typically have a volume of about 40 to about 60 cc, a thickness of about 13 mm to about 16 mm and a mass of approximately 100 grams.
It is desirable to reduce the volume, thickness and mass of such capacitors and ICD IPGs without reducing deliverable energy. Doing so is beneficial to patient comfort and minimizes complications due to erosion of tissue around the ICD IPG. The size of the ICD IPG is commonly measured in terms of its volume, i.e., displacement. The volume is determined largely by the size and arrangement of the major components enclosed within an IPG xe2x80x9ccanxe2x80x9d or hermetically sealed housing and the size of a connector header mounted to the IPG housing for making electrical connection with ICD leads. The major components within the ICD IPG housing include one or more battery, one or more high voltage capacitor, electronic modules, a telemetry antenna, a large internal discharge resistor (in early ICD IPGs), and any plastic frame or skeleton, spaces or liners supporting these components within the can. Also, the volume of the interconnection wiring between these components can be appreciable.
The high voltage capacitor(s) are among the largest volume components that must be enclosed within the ICD IPG housing. Thus, a great deal of effort has been expended in decreasing the volume of the capacitor(s) to allow for the balanced addition of volume to the battery, thereby increasing longevity of the ICD IPG, or balanced addition of new components, thereby adding functionality to the ICD IPG or to decrease the volume of the ICD IPG housing.
Various types of flat and spiral-wound capacitors are known in the art, some examples of which are described as follows and/or may be found in the patents listed in Table 1 of commonly assigned U.S. Pat. No. 6,006,133. Typically, an electrolytic capacitor is fabricated with a capacitor case enclosing a xe2x80x9cvalve metalxe2x80x9d (e.g., aluminum) anode layer (or xe2x80x9celectrodexe2x80x9d), a valve metal (e.g. aluminum) cathode layer (or xe2x80x9celectrodexe2x80x9d), and a Kraft paper or fabric gauze spacer or separator impregnated with a solvent based liquid electrolyte interposed therebetween. The aluminum anode layer is typically fabricated from aluminium foil that is first etched and then xe2x80x9cformedxe2x80x9d by passage of electrical current through the anode layer to oxidize the etched surfaces so that the aluminium oxide functions as a dielectric layer. The electrolyte comprises an ion producing salt that is dissolved in a solvent and provides ionic electrical conductivity between the cathode layer and the aluminum oxide dielectric layer. The energy of the capacitor is stored in the electromagnetic field generated by opposing electrical charges separated by the aluminum oxide layer disposed on the surface of the anode layer and is proportional to the surface area of the etched aluminum anode layer. Thus, to minimize the overall volume of the capacitor one must maximize anode surface area per unit volume without increasing the capacitor""s overall (i.e., external) dimensions. The separator material, anode and cathode layer terminals, internal packaging, electrical interconnections, and alignment features and cathode material further increase the thickness and volume of a capacitor. Consequently, these and other components in a capacitor and the desired capacitance limit the extent to which its physical dimensions may be reduced.
Some ICD IPGs employ commercial photoflash capacitors similar to those described by Troup in xe2x80x9cImplantable Cardioverters and Defibrillators,xe2x80x9d Current Problems in Cardiology, Volume XIV, Number 12, December 1989, Year Book Medical Publishers, Chicago, and as described in U.S. Pat. No. 4,254,775. The electrodes or anode and cathodes are wound into anode and cathode layers separated by separator layers of the spiral. Most commercial photoflash capacitors contain a core of separator paper intended to prevent brittle, highly etched aluminum anode foils from fracturing during winding of the anode, cathode, and separator layers into a coiled configuration. The cylindrical shape and paper core of commercial photoflash capacitors limits the volumetric packaging efficiency and thickness of an ICD IPG housing made using same.
The early ICD IPG depicted in the ""778 patent is much larger in volume and weight than current ICD IPGs due to use of such large cylindrical capacitors as well as large volume batteries, large discrete electrical components, circuit boards and the supporting skeleton for these components. The capacitors are supported and electrically isolated from one another and the batteries and the electrical circuitry by end cups and spacers and are cushioned from the housing itself by electrical insulating tape wound about the capacitors and batteries. An appreciable amount of unfilled space appears to remain in the ICD IPG housing. Moreover, tedious hand assembly appears to have been necessary to assemble these components. Nevertheless, similar component arrangements and assembly techniques have continued to be used until recently with more compact integrated circuit modules as evidenced by the ICD IPG depicted in U.S. Pat. Nos. 5,741,313, 5,749,910, 5,814,090, and 6,026,325, for example.
Recently developed ICD IPGs employ one or more flat or xe2x80x9cprismaticxe2x80x9d, high voltage, electrolytic capacitor to overcome some of the packaging and volume disadvantages associated with cylindrical photoflash capacitors. Flat aluminum electrolytic capacitors for use in ICD IPGs have been disclosed, e.g., those improvements described in xe2x80x9cHigh Energy Density Capacitors for Implantable Defibrillatorsxe2x80x9d presented by P. Lunsmann and D. MacFarlane at CARTS 96: 16th Capacitor and Resistor Technology Symposium, 11-15 March 1996, and at CARTS-EUROPE 96: 10th European Passive Components Symposium., 7-11 October 1996, pp. 35-39. Further features of flat electrolytic capacitors for use in ICD IPGs are disclosed in the above-referenced ""133 patent and in U.S. Pat. Nos. 4,942,501; 5,086,374; 5,131,388; 5,146,391; 5,153,820; 5,522,851, 5,562,801; 5,628,801; and 5,748,439, all issued to MacFarlane et al. For example, U.S. Pat. Nos. 5,131,388 and 5,522,851 disclose a flat aluminium electrolytic capacitor having a plurality of stacked capacitor layers each comprising an xe2x80x9celectrode stack sub-assemblyxe2x80x9d. Each capacitor layer contains one or more anode sheet forming an anode layer having an anode tab, a cathode sheet or layer having a cathode tab and a separator for separating the anode layer from the cathode layer. The electrode stack sub-assembly is fitted into a sealed capacitor housing filled with electrolyte. One of the sets of anode tabs or cathode tabs, typically the anode cathode tabs, are coupled to an electrical feedthrough pin extending through the capacitor housing, and the other of the set of anode tabs or cathode tabs, typically the cathode tabs, are electrically connected to the capacitor housing, whereby the cathode housing is active.
The capacitor housing is shaped and dimensioned to snugly fit within and fill the volume within a particular portion of the ICD IPG housing. Early pacemaker IPGs were disk shaped and informally referred to as xe2x80x9chockey pucksxe2x80x9d due to their similar diameter, thickness, and weight. Current pacemaker and ICD IPG housings are dramatically reduced in size, volume, and weight but retain the resemblance in certain respects. The housings of ICD IPGs as well as other IMDs that are to be implanted subcutaneously in the pectoral region continue to typically have opposed, nominally planar major surfaces joined by a continuous side wall that is typically flat along one segment of the edge to be fitted with or support the ICD IPG connector header and a filtered electrical feedthrough assembly. The remaining sidewall is rounded at the edges with the opposed, generally planar major surfaces so as to eliminate sharp 90xc2x0 edges presented to subcutaneous tissue.
The flat electrolytic capacitor housings (as well as the battery housings) are shaped to fit the designated portions of the IPG housing. Therefore, the capacitor cases typically have generally opposed, substantially planar, major case sides joined by a continuous minor case side that maintains the opposed, substantially planar, major case sides substantially in parallel alignment, the minor case side defining the nominal height or thickness of the capacitor case. The capacitor case typically comprises a cover and a xe2x80x9ccanxe2x80x9d, each formed of a conductive housing material. The can typically has a top opening or a side opening to enable insertion of an electrode stack assembly of one of the types described above conforming in shape to the space within the can. The anode and cathode layers of the electrode stack assembly are electrically connected to anode and cathode terminals extending through the can, although the cathode layers can simply be coupled to the can. The case cover is welded to the can after anode and cathode terminal electrical connections are made, and electrolyte is injected into the space within the can through a fill port that is then closed. The resulting capacitor is electrically tested and aged, and some convex bulging of the substantially planar, major case sides can occur due to internal pressures. One approach to the packaging of the components of an ICD IPG including a single flat electrolytic capacitor is disclosed in U.S. Pat. No. 5,370,669. The flat batteries, electrolytic capacitor, and an electronic circuit module are sandwiched together using polymeric spacers, clips, shells, and retainers and frames. The capacitor housing is insulated on one major flat surface from the circuit module by a flat polyimide spacer and fitted into a polymeric retainer so that the other major surface and edge of the capacitor housing is electrically insulated from the ICD IPG housing and batteries.
Complex retainers and spacers are also employed in the above-referenced ""910 patent to retain and electrically isolate the cylindrical capacitors from the other components of the ICD IPG. An electrical grounding shield within an outer liner is also interposed between the circuit module and other circuit interconnections and the ICD IPG housing to attenuate electromagnetic coupling and facilitate use of the ICD IPG housing as a cardioversion/defibrillation shock delivery electrode. The above-referenced ""090 patent employs a heat shrinkable outer liner and grounding shield that is heat shrunk over the assembled components that are fitted into the ICD IPG housing.
A set of equivalent volume and capacitance flat or prismatic electrolytic capacitors having active capacitor cases are employed in many current ICD IPGs that are electrically connected in series to be charged to a high voltage as described in the commonly assigned above-referenced ""133 patent. The capacitors can be made to be thin and mounted side-by-side. However, it is necessary to electrically insulate the facing sides of the electrically active capacitor cases from one another and other surfaces of the capacitor cases from the IPG housing and other electrical components. Therefore, the set of electrolytic capacitors are typically assembled into a capacitor sub-assembly having insulating layers between the capacitor sides facing one another and around the exposed surfaces of the set of capacitors. Such insulation packaging undesirably increases the bulk of the capacitor sub-assembly and the resulting bulk of the IPG housing.
One way of insulating the facing sides of the capacitor cases is to adhere an electrically insulating sheet between them. In one approach, a pre-cut, flat, planar sheet of a polymer, e.g., polyimide, is die cut into the overall shape of the capacitor sides with extensions adapted to be adhered to the side of the capacitor set. The polymer sheet has pressure sensitive adhesive layers deposited on each opposed surface that are protected by carrier papers until the sheet is used. In use, the carrier papers are removed, and the insulating sheet is carefully adhered to the capacitor sides and the extensions are folded over the capacitor case edges. Then, if any exposed insulating sheet remains, it can be manually trimmed away.
The pre-cut shape of the planar sheet does not conform to the features of the capacitor case major sides and the major side perimeters to be insulated from one another. The application of the insulating sheet must be carefully accomplished to avoid wrinkles or bunches and buckling when the perimeter of the planar sheet is wrapped around the radius. While the capacitor case major sides are nominally planar and parallel to one another, they can bulge out somewhat within prescribed tolerances as described above, leading to uneven spacing between the facing capacitor sides. The capacitor can is formed with a nominally 90xc2x0 radius, in the bend at the perimeter of the capacitor can bottom (one of the capacitor case major sides) where the can bottom is joined with the capacitor case minor side in the can forming process. Wrinkling of the perimeter of the insulating sheet over the radius can cause edge buckling or bunching of the insulating sheet and can increase the overall volume of the capacitor sub-assembly, even if these irregularities are within defined tolerances. Consequently, it is necessary to set the dimensional tolerances of the capacitor sub-assembly and specify dimensions of the IPG housing and the capacitor retainer or spacer, if any, to be large enough to accommodate capacitor cases within the tolerances, adding to the bulk of the IPG housing. Moreover, capacitor sub-assemblies that fail to meet the tolerances must be reworked or scrapped, adding to expense.
In another approach, an insulating layer of vapor-deposited thin insulating film, (e.g., Parylene) or have been deposited on the sides of the capacitor case. This approach leads to inconsistent insulation and higher cost, higher manufacturing equipment maintenance, longer manufacturing process cycle times, etc.
Certain ICD IPGs have been powered by two batteries that have conductive cases that must be electrically isolated from mutual contact or from contact with the electronic circuit module or the capacitor case(s) as illustrated in the ""669 patent. Relatively bulky polymeric retainers and spacers are also required to maintain this electrical isolation.
Thus, there is a need for further reducing capacitor sub-assembly volume, increasing capacitor sub-assembly dimensional uniformity, and reducing cost and complexity of the capacitor sub-assembly manufacturing process for such capacitor sub-assemblies used in ICD IPGs and other IMDs. There are similar continuing needs in the fabrication of battery sub-assemblies used in ICD IPGs and other IMDs.
The present invention provides for methods and apparatus for forming a component sub-assembly for assembly with other components of an ICD IPG into an IPG housing where the component sub-assembly comprises two or more components stacked side-by-side such that the facing component case major sides are separated by a reliable and simple to apply insulation layer of minimal thickness. Preferably, one or both of the exposed component case major sides are also insulated by a reliable and simple to apply insulation layer of minimal thickness.
In accordance with a first aspect of the present invention, a shape conforming insulating spacer is formed of an insulating material to conform to the shape of a first component case major side and the perimeter of that major side. The shape conforming insulating spacer is cup-shaped preferably having a concave spacer bottom conforming to any convex bulge in the first component case major side and a spacer side wall or rim conforming with the bend in the perimeter of the first component case major side.
In one embodiment, the interior surface of the shape conforming insulating spacer is simply applied against the mating surface of the first component case major side and maintained there by applied pressure during the final assembly of the component sub-assembly and/or by an electrostatic surface attraction of the material or by a wetting agent that forms a vapor-lock or the like.
In a further embodiment, the interior surface of the shape conforming insulating spacer is adhered with the surface of the first component case major side. The adhesive can be applied as an adhesive layer first either to the interior surface of the shape conforming insulating spacer or to the surface of the first component case major side. The shape conforming insulating spacer and first component case major side are then brought together with the adhesive layer between them and adhering them together. Then, the adhesive can be applied as an adhesive layer first either to the exterior surface of the shape conforming insulating spacer or to the surface of the second component case major side. The shape conforming insulating spacer adhered to the first component major side and the and second component case major side are then brought together with the adhesive between them and adhering them together.
The applied adhesive can add or contribute undesirable thickness to the assembly of the two components and the shape conforming insulating spacer. Consequently, it is preferable to thin the adhesive layer or to eliminate it in bulges or irregularities of the first and/or second major sides of the component cases that are joined together.
In one preferred alternative embodiment, a band or other pattern of adhesive is applied to the interior surface of the shape conforming insulating spacer such that a central portion of the interior and exterior surfaces of the spacer bottom are only thinly coated with adhesive or are substantially free of adhesive to minimize the thickness contribution of the adhesive. In this way, the thickness contribution of the adhesive layer in locations where any bulging of the facing component sides is likely to occur is minimized. The adhesive can be a liquid or self-levelling adhesive e.g., a contact cement or the like and is sprayed or painted all over the selected surfaces or in the above-described patterns on the selected surfaces.
Preferably the adhesive layer is formed of a pressure sensitive adhesive (PSA) layer applied manually or by other means to such surfaces in any of the orders described herein. The PSA layer preferably is provided in the form of a free-standing sheet of PSA protected on both sides by contact paper and cut to any of the shapes described herein. The contact paper is removed from one side of the PSA layer sheet, and the exposed PSA is adhered first to one of the surfaces to be joined. The contact paper is then removed from the other side of the PSA layer sheet, and the exposed PSA is adhered to the other of the surfaces to be joined.
The shape conforming insulating spacer can be placed between the facing major sides any further components that are stacked side-by-side in component sub-assemblies comprising three or more components.
Moreover, the shape conforming insulating spacers without any adhesive applied to the exterior surfaces of the spacer bottom can be adhered to the first component case major side at the end of the stack of components.
Advantageously, the assembly of the resulting component sub-assembly is simplified, and the possibility of errors made during assembly is reduced. The thickness of the resulting component sub-assembly is minimized and tolerances can be reduced. The thickness of the retainer, if any, for the component sub-assembly and the IPG housing can be minimized.
The present invention is preferably employed in the fabrication of, and as the resulting fabricated, capacitor sub-assemblies of a plurality of electrolytic capacitors or a battery sub-assembly of a plurality of batteries.
This summary of the invention and the advantages and features thereof have been presented here simply to point out some of the ways that the invention overcomes difficulties presented in the prior art and to distinguish the invention from the prior art and is not intended to operate in any manner as a limitation on the interpretation of claims that are presented initially in the patent application and that are ultimately granted.